This invention relates to switching circuits. More particularly, this invention relates to switching circuits having independent source and sink components.
One conventional method of switching ON an output circuit, e.g., the output transistor of a switching regulator, is to drive its base high with a fixed current source coupled to Vcc. After the circuit is switched ON, an anti-saturation circuit may be used to detect the voltage across the output circuit, and, when necessary, to shunt excess drive to ground or into the collector of the output circuit. When the output circuit is switched OFF, a fixed current sink may be used to shut OFF the output circuit by sinking current from the base of the output circuit to ground.
This approach, however, has several potential problems. One problem is that the source and sink currents required to drive the output circuit are often excessive under lighter loads. Under such conditions, power is wasted when the output circuit is switched ON by a source circuit to a greater extent than necessary, and when the output circuit is switched OFF by a sink circuit to a greater extent than necessary. In addition, the extra source current supplied by the source circuit under lighter loads may slow the switch OFF time of the source circuit. Because the output circuit will not switch OFF as long as the source circuit is still ON, a delay in switching OFF the source circuit lengthens the switch-OFF period of the output circuit.
Another potential problem with conventional switching circuits is that the sink circuit often must be switched OFF itself before the output circuit can be switched back ON. This also may add extra time delay to the switch-ON period of the output circuit.
One other problem that may occur is that additional circuits may be required to prevent the output circuit from saturating. These circuits, which typically shunt extra drive current to ground, rely on an assumed saturation voltage for reference. Thus, these circuits typically have no compensation for variations in transistor characteristic or load current.
A problem may also exist if the emitter of the output circuit is held above ground during the switch OFF period, and the sink circuit grounds the base of the output circuit. This arrangement can backward bias the emitter-base junction of the output circuit and cause an undesirable leakage path from the emitter of the output circuit through its base to ground via the sink circuit. This leakage can damage the output circuit.
Finally, if the load is short-circuited, the source circuit may be forced to provide maximum drive current. This can cause the output circuit to operate at excessively high currents, potentially causing damage to the output circuit.
Therefore, it is an object of this invention to provide a switching circuit that conserves power and reduces the switch-OFF time by limiting the excess current provided by the source and sink circuits in the switching circuit.
It is also an object of this invention to provide a switching circuit which reduces the switch-ON period by minimizing delays in switching OFF the sink circuit.
It is also an object of this invention to provide a switching circuit that compensates for variations in transistor characteristics and/or load current.
It is also an object of this invention to provide a switching circuit which is not subject to damage caused by back-biasing of the output circuit emitter-base junction.
It is also an object of this invention to provide a switching circuit which is not subject to damage when the load is short-circuited.
Therefore, switching circuits and techniques including output circuits, switch-ON circuits, switch-OFF circuits and controlled current sources are provided. A single input signal, which is either ON or OFF, is fed to both the switch-ON circuit and the switch-OFF circuit. In response to an ON signal, the switch-ON circuit provides a switch-ON current to initiate the turn-ON of the output circuit. A sample of the output current, e.g. the collector-emitter current in an output transistor acting as the output circuit, is then sampled and fed to the controlled current source which then drives the output circuit to a higher level. At this higher level, the output circuit may be switched fully ON. In response to an OFF signal, the switch-OFF circuit operates to sink drive current away from the output circuit to initiate turn-OFF of the output circuit. A sample of the output current is then fed to the controlled current source which then drives the switch-OFF circuit to a higher level. At this higher level, the switch-OFF circuit is able to fully switch OFF the output circuit.